Methods for producing seawater based, high temperature viscoelastic surfactant fluids with low scaling tendency

ABSTRACT

Embodiments of the present disclosure are directed to a method of producing a viscoelastic surfactant (VES) fluid, the VES fluid comprising desulfated seawater. The method of producing the VES fluid comprises adding an alkaline earth metal halide to seawater to produce a sulfate precipitate. The method further comprises removing the sulfate precipitate to produce the desulfated water. The method further comprises adding a VES and one or more of a nanoparticle viscosity modifier or a polymeric modifier to the desulfated seawater. Other embodiments are directed to VES fluids that maintain a viscosity greater than 10 cP at temperatures above 250° F.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 15/661,596 filed Jul. 27, 2017, the entiredisclosure of which is hereby incorporated herein by reference.

BACKGROUND Technical Field

This disclosure relates to fluids used in fluid injection operations.More specifically, this disclosure relates to viscoelastic surfactantfluids used in fluid injection operations including hydraulic fracturingoperations.

Background

Viscoelastic surfactant (VES) molecules form elongated, flexible,wormlike micelles in solution. These wormlike micelles formthree-dimensional networks that cause limited motility of watermolecules, resulting in a viscoelastic gel. The resulting viscoelasticgels, also known as a VES fluids, have long been used in hydrocarbonwellbore-related fluid injection operations such as fracturing,completion, acidizing, sand control, and water shutoff. VES fluidsexhibit superior proppant suspending, carrying ability, and very lowformation damage compared to other treatment fluids making themfavorable for fluid injection operations.

In certain hydrocarbon wellbore-related operation conditions, freshwater is not readily available to produce VES fluids. For example, deepsea oil drilling operations and some coastal drilling operations havelimited access to fresh water, but access to an abundance of seawater.In such situations, it may be desirable to make VES fluids with seawaterrather than importing large quantities of fresh water. But, fluidsproduced with natural seawater may be problematic for use in injectionoperations due to the presence of incompatible ions.

Seawater contains ions that are incompatible with formation waters, thatis, the water in the earth surrounding a wellbore. Incompatibilitybetween injection fluids and formation waters can cause scaling inwellbores and on drilling equipment. Seawater contains several ionsincluding, but not limited to, calcium, magnesium, sodium, chloride, andsulfate. Formation water, the water surrounding the wellbore, oftencontains barium (Ba) and strontium (Sr). Seawater could have a sulfateconcentration greater than 4,000 milligrams per liter (mg/L). Whenconventional seawater-based VES fluids are used in water injectionoperations, the sulfate in the seawater reacts with barium ions in theformation water. This reaction produces a solid sulfate precipitatessuch as BaSO₄,SrSO₄, or combinations thereof. The precipitated sulfatescan buildup in wellbores and on drilling equipment; this buildup is alsoknown as scaling.

Scaling can cause the shutdown of wellbores and shorten wellborelifetime. Current solutions to scaling involve treatments after thescaling occurs. These treatments can increase operating costs andpermanently decrease wellbore productivity. Scale remediation is lessdesirable than scale prevention. First, remediation techniques maydamage the wellbore and permanently decrease wellbore productivity andshorten wellbore life. Second, stopping a drilling operation to repairscale damage can exacerbate the scale damage. A stopped wellbore losespressure and cools down, precipitating even more sulfate scales.Therefore, preventing scaling is critical to operating an efficientwellbore in an environment without excess freshwater.

Further, current VES fluids made from seawater operate at a maximumtemperature of approximately 250 degrees Fahrenheit (° F.). Attemperatures greater than 250° F. the viscosities of conventional VESfluids could drop below 10 centipose (cP) to lose the designedfunctions. VES fluids with a viscosity below 10 cP (at a shear rate of100 s⁻¹) may be unsuitable for fluid injection operations.

SUMMARY

Accordingly, there exists a need for a method to create a VES fluidusing seawater that prevents scaling from occurring. In order to preventscaling from occurring, embodiments of the present disclosure desulfateseawater for use in the production of fluids for fluid injectionoperations. Here, to desulfate means to remove all or part of thesulfate ions from a water sample. The desulfated seawater can be used inthe production of VES fluids. The VES fluids made with desulfatedseawater have a relatively low sulfate concentration, inhibiting theformation of BaSO₄ and SrSO₄ scaling.

Embodiments of the present disclosure are directed to a method ofproducing a viscoelastic surfactant (VES) fluid, the VES fluidcomprising desulfated seawater. The method of producing the VES fluidcomprises adding an alkaline earth metal halide to seawater to produce asulfate precipitate. The method further comprises removing the sulfateprecipitate to produce the desulfated water. The method furthercomprises adding a VES and one or more of a nanoparticle viscositymodifier or a polymeric viscosity modifier like a polyacrylamide oracrylamide copolymer viscosity modifier to the desulfated seawater.

VES fluids should maintain a viscosity of at least above about 10 cP tobe suitable for fluid injection operations. Conventional VES fluidsdecrease in viscosity to below 10 cP at temperatures greater than 250°F. There exists a need for a VES fluid comprising seawater that canmaintain a viscosity above about 10 cP at a temperature above 250° F.,above 300° F., or above 350° F.

Embodiments of the present disclosure are also directed towards VESfluids that maintain a viscosity of greater than 10 cP at temperaturesabove 250° F. The VES fluids may comprise a viscoelastic surfactant,desulfated seawater, and at least one stabilizing calcium salt. The VESfluids may further comprise a nanoparticle viscosity modifier or apolymeric like polyacrylamide viscosity modifier.

Additional features and advantages of the described embodiments will beset forth in the detailed description which follows, and in part will bereadily apparent to those skilled in the art from that description orrecognized by practicing the described embodiments, including thedetailed description which follows, the claims, as well as the appendedfigures

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparison of four photographs of natural seawater anddesulfated seawater mixed with formation water: (a) shows a solutionthat is 80 percent by total volume (vol. %) of desulfated seawater and20 vol. % formation water that had been resting at 300° F. for one day;(b) shows a solution that is 80 vol. % natural seawater and 20 vol. %formation water that had been resting at 300° F. for one day; (c) showsa solution that is 80 percent by total vol. % desulfated naturalseawater and 20 vol. % formation water that had been resting at 300° F.for seven days; (d) shows a solution that is 80 percent by total vol. %natural seawater and 20 vol. % formation water that had been resting at300° F. for seven days.

FIG. 2 is a graphical depiction of viscosity-temperature curveillustrating: (i) a VES fluid without a nanoparticle viscosity modifieror polymeric viscosity modifier and (ii) a VES fluid comprising about 79pounds of an anionic polyacrylamide polymer per thousand gallons (ppt)of VES fluid.

The embodiments set forth in the drawings are illustrative in nature andnot intended to be limiting to the claims. Moreover, individual featuresof the drawings will be more fully apparent and understood in view ofthe detailed description.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to VES fluids for usein hydrocarbon wellbore-related fluid injection operations. Fluidinjection operations include treatments of underground oil and gasbearing formations, such as fracturing, completion, acidizing, sandcontrol, and water shutoff. The present disclosure generally relates toVES fluids comprising seawater and to methods of their formulation. Thisdisclosure describes a VES fluid that maintains a suitable viscosity attemperatures greater than 250° F. In the context of this disclosure, asuitable viscosity is a viscosity greater than or equal to about 10 cPas measured by the American Petroleum Institute Recommended Practice 39(“API RP 39”) entitled “Recommended practices on measuring the viscousproperties of a cross-linked water-based fracturing fluid.” To measurethe viscosity of a VES fluid sample under the conditions designed tosimulate those in a high temperature and high pressure subterraneanreservoir formation, 52 mL of the fluid sample was placed into aFann50-type viscometer such as Grace M5600 HPHT rheometer. Tests wereperformed at the bottomhole temperature, following the API RP 39schedule and under about 400 psi of nitrogen. The bottomhole temperaturerefers to the temperature in the borehole at total depth at the time itis measured. The API RP 39 schedule consisted of continuous fluidshearing at 100/s shear rate and a series of shearing ramps at 100, 75,50, 25, 50, 75, and 100/s once the fluid temperature was within 5° F. ofthe bottomhole temperature and occurring periodically for every 30minutes.

In one embodiment, a method may include producing a viscoelastic fluidcomprising desulfated seawater. The method may comprise adding analkaline earth metal halide to seawater to produce a sulfateprecipitate. The method may further comprise removing the sulfateprecipitate to produce desulfated seawater. The method may furthercomprise adding a viscoelastic surfactant, and one or more of ananoparticle viscosity modifier or a polymeric viscosity modifier to thedesulfated seawater. The viscoelastic surfactant may have a generalformula (I):

where R₁ is a saturated or unsaturated hydrocarbon group of from 17 to29 carbon atoms, R₂ and R₃ are each independently selected from astraight chain or branched alkyl or hydroxyalkyl group of from 1 to 6carbon atoms, R₄ is selected from H, hydroxyl, alkyl or hydroxyalkylgroups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m isan integer of from 1-20; and n is an integer of from 0-20.

The calcium ions that remain in solution from the desulfation processsurprisingly stabilize the viscoelastic surfactant three-dimensionalmicelle network and maintain a suitable viscosity of at least 10 cP attemperatures greater than or equal to 250° F.

In one or more embodiments, the VES with the general formula I has an R₁that is a saturated or unsaturated hydrocarbon group of from 17 to 29carbon atoms. In other embodiments, R₁ is a saturated or unsaturated,hydrocarbon group of 18 to 21 carbon atoms. R₁ may be restricted to asingle chain length or may be of mixed chain length such as those groupsderived from natural fats and oils or petroleum stocks. The natural fatsand oils or petroleum stocks may comprise tallow alkyl, hardened tallowalkyl, rapeseed alkyl, hardened rapeseed alkyl, tall oil alkyl, hardenedtall oil alkyl, coco alkyl, oleyl, erucyl, soya alkyl, or a combinationthereof.

In one embodiment, the general formula I of the viscoelastic surfactant,R₂ and R₁ are each independently selected from a straight chain orbranched alkyl or hydroxyalkyl group of from 1 to 6 carbon atoms, inother embodiments from 1 to 4 carbon atoms, and in another embodimentfrom 1 to 3 carbon atoms. R₄ is selected from H, hydroxyl, alkyl orhydroxyalkyl groups of from 1 to 4 carbon atoms, and can be selectedfrom methyl, ethyl, hydroxyethyl, hydroxyl or methyl, but is not limitedto this list of groups.

The general formula I has the variables k, m, and n. In one embodiment,k is an integer of from 2-20, in other embodiments, from 2 to 12, and inanother embodiment from 2 to 4. The m is an integer of from 1-20, inother embodiments from 1 to 12, in another embodiment from 1 to 6, andin some embodiments, m can also be an integer from 1 to 3. Finally, n isan integer from 0 to 20, from 0 to 12, or from 0 to 6. In someembodiments, n is an integer from 0 to 1.

In one embodiment, the VES is erucamidopropyl hydroxypropylsultaine,commercially available as Armovis EHS, provided by Akzo Nobel. Variousamounts of VES in the VES fluid are contemplated. In one or moreembodiments, the viscoelastic surfactant fluid comprises from 0.1 vol. %to 20 vol. % viscoelastic surfactant based on the total volume of theVES fluid. In other embodiments, the viscoelastic surfactant fluidcomprises from 0.01 vol. % to 30 vol. %; from 0.1 vol. % to 25 vol. %;from 0.1 vol. % to 10 vol. %; from 0.1 vol. % to 5 vol. %; from 1 vol. %to 10 vol. %; or from 1 vol. % to 5 vol. %.

The viscoelastic surfactant fluid may further comprise desulfatedseawater and at least one stabilizing calcium salt. The viscoelasticsurfactant fluid may further comprise one or more of a nanoparticleviscosity modifier or a polymeric viscosity modifier. The nanoparticleviscosity modifier may have a particle size of 0.1 to 500 nanometers(nm). The polymeric viscosity modifier contains polyacrylamide oracrylamide copolymer viscosity modifier that may have a weight averagemolecular weight (Mw) of from 250,000 grams per mole (g/mol) to40,000,000 g/mol.

The addition of an alkaline earth metal halide to seawater increases thecalcium concentration of the seawater and will produce a sulfateprecipitate. Suitable alkaline earth metal halides include, by way ofnon-limiting example, CaCl₂, CaBr₂, or CaI₂. In one embodiment, thealkaline earth metal halide is added to a final concentration of from 5percent by total weight (wt. %) to 50 wt. % based on the total weight ofthe desulfated seawater solution. In other embodiments, the alkalineearth metal halide may be added to a final concentration of from 10 wt.% to 40%; from 10 wt. % to 30 wt. %; from 20 wt. % to 50 wt. %; or from30 wt % to 40 wt. % based on the total weight of the desulfated seawatersolution.

Alkaline earth metal halides have a generally high solubility in water.For example, CaCl₂ has a maximum solubility of 745 grams per liter (g/L)in 20° C. water. By contrast, CaSO₄ has a relatively low solubility inwater. Calcium sulfate has a maximum solubility of 2.1 g/L in 20° C.water. The relative solubilities of CaCl₂ and CaSO₄ mean that whenenough CaCl₂ is added to a solution containing sulfate ions, CaSO₄ willfall out of solution, or precipitate. Chemical equation 1 shows thischemistry.CaCl₂ _((s)) +SO₄ _((aq)) ²⁻→CaSO₄ _((s)) +2Cl_((aq)) ⁻  Eq. (1)

Another way to compare the relative solubilities of CaCl₂ and CaSO₄ isto look at their solubility expressions and solubility productconstants. The solubility product constant (K_(sp)) is the equilibriumconstant for a substance dissolving in an aqueous solution. The higherthe K_(sp) of a substance, the more soluble the substance. K_(sp) can beexpressed as the product of the concentration of the dissociated ions ina saturated solution. For example, the K_(sp) of CaSO₄ could beexpressed as K_(sp)═[Ca²⁺][SO₄ ²⁻] where [Ca²⁺] and [SO₄ ²⁻] representthe ion concentrations at saturation.

In one or more embodiments, CaO is added with the alkaline earth metalhalide. The added CaO increases the Ca²⁺ concentration of the seawater,therefore precipitating out more CaSO₄. The dissociation of CaO is alsoan exothermic reaction; the heat generated from the reaction is alsouseful in facilitating the dissolution of calcium ions.

The removal of CaSO₄ precipitate is necessary to produce desulfatedwater. In one embodiment, the CaSO₄ precipitate is removed byfiltration. In other embodiments, the CaSO₄ precipitate is removed byone or more of filtration, centrifuging, flocculation, agglomeration,coagulation, or coalescence.

A method of producing a viscoelastic surfactant fluid comprisingdesulfated seawater may further comprise adding a VES and one or more ofa nanoparticle viscosity modifier or a polymeric modifier includingpolyacrylamide viscosity modifier to desulfated seawater. Generally, VESfluids break down and lose viscosity at temperatures greater than 250°F. VES fluids are viscous because of a three dimensional VES micellestructure throughout the fluid. At temperatures above 250° F., thekinetic energy of the VES fluid system increases to the point where thethree dimensional VES micelle network loses thermodynamic stability. Inembodiments, the VES of general formula (I) together with one or more ofa nanoparticle viscosity modifier or a polymeric viscosity modifierstabilize the three dimensional VES micelle network at temperaturesgreater than 250° F.

By removing the sulfate from the seawater, the water can be used in theformulation of VES fluids without the costs and risks of scaling. Byadding excess alkaline earth metal halide to the seawater, any sulfatesin solution will associate with the calcium and precipitate out. Thissolid CaSO₄, once precipitated, can be removed from the water. Thedesulfated water can then be used in a VES fluid without causing scalingwhen introduced to the formation water.

Not all calcium added to the saltwater is consumed in the associationreaction with sulfate. An excess of calcium remains in solution. Thiscalcium chloride serves a second function as stabilizing the VES. Athigh temperatures, the wormlike micelles formed by the VES are morestable with alkali earth metals, such as calcium. Without being limitedby theory, it is believed the calcium ions can strengthen the threedimensional micelle network.

The viscosity of a viscoelastic fluid may vary with the stress or rateof strain or shear rate applied. In the case of shear deformations, itis very common that the viscosity of the fluid drops with increasingshear rate or shear stress. This phenomenon is known as “shearthinning.” Surfactants can cause viscoelasticity in fluids and maymanifest shear thinning behavior. For example, when viscoelastic fluidis passed through a pump or is in the vicinity of a rotating drill bit,the viscoelastic fluid is in a higher shear rate environment and theviscosity is decreased, resulting in low friction pressures and pumpingenergy savings. When the shear stress is removed, the fluid returns to ahigher viscosity condition, functioning to transport and place proppant.

At elevated temperatures, the average kinetic energy of the molecules inthe fluid increases, causing more disruptions to the VES micellestructures and the attractions among the micelles. This can lower theoverall viscosity of the fluid. Generally speaking, an increase intemperature correlates to a logarithmic decrease in the time needed toimpart equal strain under a constant stress. In other words, it takesless work to stretch a viscoelastic material an equal distance at ahigher temperature than it does at a lower temperature. The presence ofan alkali calcium stabilizing salt in combination with the addition ofone or more of nanoparticles and polymers to the fluid may furtherimprove the VES fluid viscosity at elevated temperatures.

In one or more embodiments, the VES fluid may be enhanced withnanoparticle viscosity modifiers to increase the viscosity of the VESfluid. Without being limited by theory, it is believed the selectednanoparticles associate with the VES micelles to more efficiently form athree dimensional micelle network. The presence of the selectednanomaterials could render enhanced interconnected and aggregatedmicellar morphologies in the VES fluids. The VES-enhancing additivesmight have, through van der Waals forces, simultaneously attached tomultiple VES micelles in the fluids, thus strengthening the3-dimensional network of the VES micelles. This way, the overall fluidviscosity could be increased. The addition of nanoparticle viscositymodifiers increases the viscosity of the VES fluid and lowers the amountof VES needed.

Nanoparticle viscosity modifiers include nanomaterials, that is,materials having unit sizes on the scale of one to one hundrednanometers (nm). Nanomaterials include, by way of non-limiting example,nanoparticles, nanotubes, nanorods, nanodots, or combinations thereof.In one or more embodiments, a nanoparticle viscosity modifier comprisescarbon nanotubes, carbon nanorods, ZnO, ZrO₂, TiO₂, compounds of Zr, Ti,Ce, Al, B, Sn, Ca, Mg, Fe, Cr, Si, or combinations thereof. In someembodiments, the nanoparticle viscosity modifier may be nonpolymeric.

In one or more embodiments, the VES fluid comprises from 0.001 wt. % to5 wt. % nanoparticle viscosity modifier. In other embodiments, the VESfluid comprises from 0.01 wt. % to 3 wt. %; from 0.01 wt. % to 1.5 wt.%; from 0.01 wt. % to 1 wt. %; from 0.03 wt. % to 1 wt. %; or from 0.05wt. % to 1 wt. % nanoparticle viscosity modifier.

Polymeric viscosity modifiers may comprise polyacrylamide homopolymersor copolymers with near zero amounts of acrylate groups; apolyacrylamide polymer or copolymer with a mixture of acrylate groupsand acrylamide groups formed by hydrolysis; or copolymer comprisingacrylamide, acrylic acid, or other monomers.

A polyacrylamide has functional groups selected from at least one memberof a group comprising carboxylate, sulfate, sulfonate, phosphate, orphosphonate. The substituted polyacrylamide may have more than onefunctional group selected from at least one member of a group comprisingcarboxylate, sulfate, sulfonate, phosphate, or phosphonate.

In one or more embodiments, the VES fluid may be enhanced with polymericviscosity modifiers to increase the viscosity of the VES fluid. Withoutbeing limited by theory, it is believed the selected polyacrylamides mayhave interacted with multiple VES micelles in the fluid, through van derWaals forces. This strengthens the three dimensional network of VESmicelles. A stronger three dimensional network results in an increasedviscosity. The addition of polymeric viscosity modifiers increases theviscosity of the VES fluid and lowers the amount of VES needed.

In one or more embodiments of this disclosure, the polymeric viscositymodifier has a weight averaged molecular weight (Mw) of from 250,000g/mol to 40,000,000 g/mol. In other embodiments, the polymeric viscositymodifier has a Mw of from 2,000,000 g/mol to 8,000,000 g/mol.

In some embodiments, additional surfactants may be added to the VESfluid. Adding additional surfactant or surfactants may increase theviscosity or enhance the three dimensional micelle network at varyingtemperatures, pressures, or other various wellbore conditions. Suchsurfactants, by way of non-limiting example, may include cationicsurfactants, anionic surfactants, nonionic surfactants, amphotericsurfactants, zwitterionic surfactants, or combinations thereof.

EXAMPLES

In the subsequent examples, VES fluids are made from either naturallyoccurring seawater or desulfated seawater. The naturally occurringseawater was obtained from the Arabian Gulf. Seawater from otherlocations worked similarly. The dissolved solids content of thenaturally occurring seawater is listed in Table 1.

TABLE 1 Solute Concentration Boron <1 mg/L Barium <1 mg/L Calcium 618mg/L  Iron <1 mg/L Magnesium 2,108 mg/L   Potassium 595 mg/L  Silicon <1mg/L Sodium 18,451 mg/L     Strontium 11 mg/L Chloride 30,694 mg/L   Sulfate 4,142 mg/L   Carbonate <1 mg/L Bicarbonate 150 mg/L  TotalDissolved Solids 56,800 mg/L   

The desulfated seawater was produced by adding calcium chloride tonaturally occurring seawater. CaCl₂ was added to the seawater at a rateof 0.334 kilograms of CaCl₂ per liter of the CaCl₂-added seawater (i.e.,per liter of the seawater volume after the addition of the CaCl₂). Theresulting solution would have a Ca²⁺ concentration of 3.02 M if therehad been no sulfate ions in the seawater. From the calculation shownbelow, only a very small portion of Ca ions were “consumed” to formCaSO₄ precipitate, while most of the Ca ions were still in the solutionwith a concentration very close to 3 M.

According to some references (for example, D. R. Linde (ed.), “CRCHandbook of Chemistry and Physics”, 83rd Edition, CRC Press, 2002), theK_(sp) of CaSO₄ is 4.93×10⁻⁵M², where K_(sp)═[Ca²⁺][SO₄ ²⁻]. Accordingto the solubility expression of CaSO₄, if the concentration of Ca²⁺ isgreater than 3 M, then the concentration of SO₄ ²⁻ cannot exceed1.63×10⁻⁵ M. Therefore, the desulfated seawater solution with a calciumconcentration greater than 3 M must have a sulfate concentration lessthan 1.64×10⁻⁵ M (or less than about 2 mg/L of sulfate). This iscompared to naturally occurring seawater which, according to Table 1,has a calcium concentration of approximately 1.54×10⁻² M and a sulfateconcentration of approximately 4.31×10⁻² M.

Example 1 and Comparative Example A comprise formation water. That is,water taken from the formation surrounding a wellbore. The dissolvedsolids content of the formation water is detailed in Table 2.

TABLE 2 Solute Concentration Sodium 78,100 mg/L  Potassium 3,889 mg/LMagnesium 1,830 mg/L Calcium 24,900 mg/L  Strontium 2,000 mg/L Barium3,832 mg/L Chloride 160,000 mg/L  Sulfate   190 mg/L

Example 1

Example 1 was prepared by combining 80 vol. % desulfated seawater with20 vol. % formation water. The resulting solution was left to rest forseven days at 300° F. The solution was photographed on the first day andon the seventh day. The photograph from the first day is shown in FIG.1A and the photograph from the seventh day is shown in FIG. 1C.

Comparative Example A

Comparative Example A was prepared by combining 80 vol. % naturalseawater (not treated) with 20 vol. % formation water. The resultingsolution was left to rest for seven days at 300° F. The solution wasphotographed on the first day and on the seventh day. The photographfrom the first day is shown in FIG. 1B and the photograph from theseventh day is shown in FIG. 1D.

As can be seen from FIG. 1, a white precipitate appeared in ComparativeExample A, but not in Example 1. The amount of precipitate inComparative Example A increased over seven days, while no precipitatedeveloped in Example 1 after seven days. This demonstrates that the useof desulfated seawater in Example 1 is capable of preventing theformation of scales like BaSO₄ scale.

The viscosities of two VES fluids made with desulfated water werecompared at 350° F. for over 110 minutes. The viscosities were measuredby API RP 39 using a Fann50-type viscometer.

Comparative Example B

Comparative Example B was prepared by combining 50 milliliters (mL) ofdesulfated seawater with 2.5 mL of VES. The viscosity of ComparativeExample B was measured with a Fann50 viscometer by API RP 39. Theseviscosity measurements are shown in FIG. 2.

Example 2

Example 2 was prepared by adding 0.5 grams (g) of a hydrophobicallymodified anionic-polyacrylamide terpolymer, commercially available underthe tradename FP9515SH by SNF Floerger, to Comparative Example B. Theviscosity of Example 2 was measured with a Fann50 viscometer by API RP39. These viscosity measurements are shown in FIG. 2.

As can be seen from FIG. 2. Example 2 had a greater viscosity thanComparative Example B over all temperatures greater than or equal to300° F. Further, the viscosity of Example 2 never dropped below 10 cP.After the first shear ramp, Example 2 had a viscosity 3.8 times that ofComparative Example B. After the second shear ramp, Example 2 had aviscosity 3.6 times that of Comparative Example B. After 60 minutes,Example 2 had a viscosity 3.0 times that of Comparative Example B. Theviscosity of the VES fluid containing the polyacrylamide terpolymermaintained a greater velocity at temperatures above 250° F. than the VESfluid without the polyacrylamide.

Example 3

Example 3 was prepared by adding 12 ppt (1 ppt=0.12 grams per liter) ofmulti-walled carbon nanotubes to Comparative Example B. The carbonnanotubes were purchased commercially from Cheap Tubes Inc. and 95percent by weight of the carbon nanotubes have a length of from 30 nm to50 nm.

The viscosity of Example 3 was measured with a Fann50-type viscometerfrom room temperature to about 350° F., at the shear rate of 100 s⁻¹.The fluid viscosity of Example 3 was about 140 cP (at the shear rate of100 s⁻¹) at about 350° F. In another test, the fluid viscosity ofComparative Example B was about 90 cP at a shear rate of 100 s⁻¹. Theviscosity enhancement due to the carbon nanotubes was therefore over 50%at 350° F.

The subject matter of the present disclosure in detail and by referenceto specific embodiments thereof, it is noted that the various detailsdisclosed within should not be taken to imply that these details relateto elements that are essential components of the various embodimentsdescribed within, even in cases where a particular element isillustrated in each of the drawings that accompany the presentdescription. Further, it will be apparent that modifications andvariations are possible without departing from the scope of the presentdisclosure, including, but not limited to, embodiments defined in theappended claims. More specifically, although some aspects of the presentdisclosure are identified as particularly advantageous, it iscontemplated that the present disclosure is not necessarily limited tothese aspects.

A first aspect of the disclosure is directed to a method of producing aviscoelastic surfactant fluid comprising desulfated seawater, the methodcomprising: adding an alkaline earth metal halide to seawater to producea sulfate precipitate; removing the sulfate precipitate to produce thedesulfated seawater; adding a viscoelastic surfactant, and one or moreof a nanoparticle viscosity modifier or a polymeric viscosity modifierto the desulfated seawater to produce the viscoelastic surfactant fluid,where the viscoelastic surfactant has the general formula:

where R1 is a saturated or unsaturated hydrocarbon group of from 17 to29 carbon atoms, R2 and R3, are each independently selected from astraight chain or branched alkyl or hydroxyalkyl group of from 1 to 6carbon atoms, R4 is selected from H, hydroxyl, alkyl or hydroxyalkylgroups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m isan integer of from 1-20; and n is an integer of from 0-20.

A second aspect of the disclosure is directed to a method of the firstaspect, where the alkaline earth metal halide comprises calcium halide.

A third aspect of the disclosure is directed to a method of the first orsecond aspect where the alkaline earth metal halide comprises CaCl₂.

A fourth aspect of the disclosure is directed to a method of any of thefirst through third aspects where the sulfate precipitate comprisesCaSO₄ precipitate.

A fifth aspect of the disclosure is directed to a method of any of thefirst through fourth aspects where the polymeric viscosity modifiercomprises polyacrylamide viscosity modifier.

A sixth aspect of the disclosure is directed to a method of any of thefirst through fifth aspects where the removing step comprisesfiltration, centrifuging, flocculation, agglomeration, coagulation,coalescence, or combinations thereof.

A seventh aspect of the disclosure is directed to a method of any of thefirst through sixth aspects where CaO is added with the alkaline earthmetal halide to produce a sulfate precipitate.

An eighth aspect of the disclosure is directed to a method of any of thefirst through seventh aspects where the viscoelastic surfactantcomprises erucamidopropyl hydroxypropylsultaine.

A ninth aspect of the disclosure is directed to a method of any of thefirst through eighth aspects where the viscoelastic surfactant fluidcomprises from 0.1 vol. % to 20 vol. % of viscoelastic surfactant.

A tenth aspect of the disclosure is directed to a method of any of thefirst through ninth aspects where the polymeric viscosity modifiercomprises nonionic polyacrylamides, acrylamide copolymers,polyacrylamide-based terpolymers, polyacrylamide-based tetra-polymers,hydrophobically modified acrylamide-based polymers, or combinationsthereof.

An eleventh aspect of the disclosure is directed to a method of any ofthe first through tenth aspects where the polymeric viscosity modifierhas a weight average molecular weight of from 250,000 g/mol to40,000,000 g/mol, more preferentially from 2,000,000 g/mol to 8,000,000g/mol.

A twelfth aspect of the disclosure is directed to a method of any of thefirst through eleventh aspects where the nanoparticle viscosity modifieris nonpolymeric.

A thirteenth aspect of the disclosure is directed to a method of any ofthe first through twelfth aspects where the nanoparticle viscositymodifier comprises carbon nanotubes, ZnO nanomaterials, ZrO₂nanomaterials, TiO₂ nanoparticles, or combinations thereof.

A fourteenth aspect of the disclosure is directed to a method of any ofthe first through thirteenth aspects where the nanoparticle viscositymodifier comprises nanosized Zr compounds, Ti compounds, Ce compounds,Zn compounds, Al compounds B compounds, Sn compounds, Mg compounds, Fecompounds, Cr compounds, Si compounds, or combinations thereof.

A fifteenth aspect of the disclosure is directed to a method of any ofthe first through fourteenth aspects where the viscoelastic surfactantfluid comprises from 0.001 wt. % to 5 wt. % nanoparticle viscositymodifier.

A sixteenth aspect of the disclosure is directed to a viscoelasticsurfactant fluid comprising a viscoelastic surfactant comprising thegeneral formula:

where R₁ is a saturated or unsaturated hydrocarbon group of from 17 to29 carbon atoms, R₂ and R₃, are each independently selected from astraight chain or branched alkyl or hydroxyalkyl group of from 1 to 6carbon atoms, R₄ is selected from H, hydroxyl, alkyl or hydroxyalkylgroups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m isan integer of from 1-20; and n is an integer of from 0-20; at least onenanoparticle viscosity modifier comprising a particle size of 0.1 to 500nanometers; desulfated seawater; and at least one stabilizing calciumsalt.

A seventeenth aspect of the disclosure is directed to a viscoelasticsurfactant fluid comprising: a viscoelastic surfactant comprising thegeneral formula:

where R₁ is a saturated or unsaturated hydrocarbon group of from 17 to29 carbon atoms, R₂ and R₃, are each independently selected from astraight chain or branched alkyl or hydroxyalkyl group of from 1 to 6carbon atoms, R₄ is selected from H, hydroxyl, alkyl or hydroxyalkylgroups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m isan integer of from 1-20; and n is an integer of from 0-20; at least onepolymeric viscosity modifier with a weight average molecular weight offrom 250,000 g/mol to 40,000,000 g/mol; desulfated seawater; and atleast one stabilizing calcium salt.

An eighteenth aspect of the disclosure includes either the sixteenth orseventeenth aspects where the viscoelastic surfactant compriseserucamidopropyl hydroxypropylsultaine.

A nineteenth aspect of the disclosure includes any of the sixteenththrough eighteenth aspects where the viscoelastic surfactant fluidcomprises from 0.1 vol. % to 20 vol. % of viscoelastic surfactant.

A twentieth aspect of the disclosure includes any of the sixteenththrough nineteenth aspects where the viscoelastic surfactant fluidcomprises from 1 wt. % to 50 wt. % of at least one stabilizing calciumsalt.

A twenty-first aspect of the disclosure includes any of the seventeenththrough twentieth aspects where the polymeric viscosity modifiercomprises nonionic polyacrylamides, acrylamide copolymers,polyacrylamide-based terpolymers, polyacrylamide-based tetra-polymers,hydrophobically modified acrylamide-based polymers, or combinationsthereof.

A twenty-second aspect of the disclosure includes any of the sixteenththrough twenty-first aspects where the polymeric viscosity modifier hasa weight average molecular weight of from 2,000,000 g/mol to 8,000,000g/mol.

A twenty-third aspect of the disclosure includes any of the sixteenth oreighteenth through twentieth aspects where the nanoparticle viscositymodifier is nonpolymeric.

A twenty-fourth aspect of the disclosure includes any of the sixteenth,eighteenth through twentieth, or twenty-third aspects where thenanoparticle viscosity modifier comprises carbon nanotubes, ZnOnanomaterials, ZrO₂ nanomaterials, TiO₂ nanomaterials, or combinationsthereof.

A twenty-fifth aspect of the disclosure includes any of the sixteenth,eighteenth through twentieth, or twenty-third through twenty-fourthaspects where the nanoparticle viscosity modifier comprises nanosized Zrcompounds, Ti compounds, Ce compounds, Zn compounds, Al compounds Bcompounds, Sn compounds, Mg compounds. Fe compounds, Cr compounds, Sicompounds, or combinations thereof.

A twenty-sixth aspect of the disclosure includes any of the sixteenth,eighteenth through twentieth, or twenty-third through twenty-fifthaspects where the viscoelastic surfactant fluid comprises from 0.001 wt.% to 5 wt. % nanoparticle viscosity modifier.

A twenty-seventh aspect of the disclosure includes any of the sixteenth,eighteenth through twentieth, or twenty-third through twenty-sixthaspects where the viscoelastic fluid has a viscosity greater than orequal to 10 cP at a temperature greater than or equal to 300° F.

Unless otherwise defined, all technical and scientific terms used inthis disclosure have the same meaning as commonly understood by one ofordinary skill in the art. The terminology used in the description isfor describing particular embodiments only and is not intended to belimiting. As used in the specification and appended claims, the singularforms “a,” “an,” and “the” are intended to include the plural forms aswell, unless the context clearly indicates otherwise.

It will be apparent to those skilled in the art that variousmodifications and variations may be made to the embodiments describedwithin without departing from the spirit and scope of the claimedsubject matter. Thus, it is intended that the specification cover themodifications and variations of the various embodiments described withinprovided such modifications and variations come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A viscoelastic surfactant fluid comprising: aviscoelastic surfactant comprising the general formula:

where R₁ is a saturated or unsaturated hydrocarbon group of from 17 to29 carbon atoms, R₂ and R₃, are each independently selected from astraight chain or branched alkyl or hydroxyalkyl group of from 1 to 6carbon atoms, R₄ is selected from H, hydroxyl, alkyl or hydroxyalkylgroups of from 1 to 4 carbon atoms; k is an integer of from 2-20; m isan integer of from 1-20; and n is an integer of from 0-20; at least onepolymeric viscosity modifier with a weight average molecular weight offrom 250,000 g/mol to 40,000,000 g/mol; desulfated seawater, where thedesulfated water is free of CaSO₄ precipitate; and at least onestabilizing calcium salt.
 2. The viscoelastic surfactant fluid of claim1, where the polymeric viscosity modifier comprises nonionicpolyacrylamides, acrylamide copolymers, polyacrylamide-basedterpolymers, polyacrylamide-based tetra-polymers, hydrophobicallymodified acrylamide-based polymers, or combinations thereof.
 3. Theviscoelastic surfactant fluid of claim 1, where the polymeric viscositymodifier has a weight average molecular weight of from 2,000,000 g/molto 8,000,000 g/mol.
 4. The viscoelastic surfactant fluid of claim 1,where the viscoelastic surfactant fluid comprises from 0.1 vol. % to 20vol. % of viscoelastic surfactant.
 5. The viscoelastic surfactant fluidof claim 1, where the viscoelastic surfactant fluid comprises from 1 wt.% to 50 wt. % of at least one stabilizing calcium salt.
 6. Theviscoelastic surfactant fluid of claim 1, where the viscoelastic fluidhas a viscosity greater than or equal to 10 cP at a temperature greaterthan or equal to 300° F.